The invention relates to a lower link draft sensing system, for an agricultural tractor having a hydraulic power lift mechanism, of the kind including two bowed rearwardly-extending leaf springs anchored to the tractor frame at their front ends, two laterally-spaced arms pivotally connected at their upper ends to the tractor frame and at their lower ends to the rear ends of the respective springs, and two lower hitch links universally connected at their front ends to the lower ends of the respective arms. The lower hitch links are adapted in conventional manner to be universally connected at their rear ends to an agricultural implement, and the draft force exerted on the tractor by the implement when in work places the springs in tension and causes them to elongate in a rearward direction. Due to the asymmetrical draft force exerted by many implements, for example ploughs, the tension in and consequent elongation of one spring often differs from the tension in and consequent elongation of the other spring. Various methods have been proposed for continuously sensing the effect of the draft force on the springs and transmitting a feed-back signal related to said force to the power lift mechanism so as to cause said mechanism to vary automatically the working depth of the implement and thereby maintain the draft force substantially constant.
In one such method, the front ends of the lower hitch links are universally connected to points at or near the respective ends of a cross-bar secured between the rear ends of the respective springs, said cross-bar being universally supported by the lower ends of the respective arms and said points being laterally offset from, that is to say either outboard or inboard of, the arms. In order to obtain a draft signal related to the average value of the tensions in the respective springs, said signal has been derived from the fore and aft displacement of the mid-point of said cross-bar. However, we have found that due to the connection of the lower hitch links to said laterally offset points on the cross-bar, the cross-bar is stressed in bending and this has a detrimental effect on the accuracy of the draft signal.
The principal object of the present invention is to avoid this disadvantage.
Another object is to provide a system of the kind referred to and having no cross-bar with means from which a draft signal accurately related to the draft force can readily be derived.